1. Field of the Invention
This invention is directed to apparatus for guiding traveling webs of material, and more particularly to a tilt frame web guiding apparatus.
2. Description of the Prior Art
It is known in the handling of long webs of material such as film, paper or fabric, in which the web is drawn over rollers from one position to another, that there is a tendency for the web to be laterally displaced from the true or preselected path of the web. Thus, where some operation is to be performed on the traveling web, e.g., the application of a coating or the printing of a design, such operations may not be uniformly applied to the web. Furthermore, the winding of the web on a roll will not be uniform if the traveling web is permitted to become randomly displaced from the centerline of the machine.
Many methods of guiding traveling webs have been utilized employing tilting frames carrying one or more guide rollers over which the traveling web passes. The principle involved in all of these web guiding machines is that the traveling web will attempt to pass across a cylindrical roller, over which it is passing, at right angles to the axis of the roller. Some machines have been constructed wherein a tilting guide frame carrying at least one cylindrical roller is moved so as to direct the traveling web back to the true path of travel, i.e., to the center of the machine. The tilting frame has been driven by various electromechanical, pneumatic, hydraulic or similar drive systems. Likewise, many web guiding machines have been constructed which utilize the tension in the web itself to move the frame and thus are generally self-actuating.
A common feature of most tilt frame web guiding machines of the prior art has been the location of the frame pivot axis near or upstream of the plane of the web entering the machine guide frame. When the web is traveling along its true path, all of the machine rollers are parallel to one another and perpendicular to the longitudinal center line of the web and machine. Also, an edge of the web or a stripe on the web parallel to the web edge is parallel to and a predetermined distance from a reference line.
All of these prior art web guiding machines have found suitable and satisfactory application in various situations. However, some of these web guiding machines have presented long standing problems in web guiding. For example, tilting of the guide frame to reposition a laterally displaced web along its true path generates a skew angle between the web exit guide roller and the succeeding machine roller which is aligned perpendicular to the machine centerline. The rotational twist of the web span exiting the guide frame causes the web to engage the next succeeding roller at a position laterally displaced from its true path or position at which it leaves the exit guide roller, resulting in a small but significant guiding error.
Another long standing problem with the conventional tilt frame-type web guiding apparatus is that the distance between the skewed web exit guide roller and a sensor sensing the web leaving the guide roller changes as the guide frame tilts. This distance change make it difficult to keep a reflective type sensor in focus on the web edge, and it can also cause the web to rub on a transmittance type sensor.
Still another problem with web edge sensing is that the web edge, due to the film twist, may curl and not lay sufficiently flat at the tangent sensing position on the web exit guide roller.
Some of the aforementioned problems of prior art guiding apparatus relating to the web and sensor geometry can be reduced by installing a transmittance sensor along the web span extending between the web exit guide roller and the guide frame and the first roller downstream of the guide frame, that is in the direction of travel of the web. The sensor generally is located at a position ranging from 50 percent to 75 percent downstream of the web span exiting from the web exit guide roller. Location of the sensor along the web span results in a decrease in control loop gain, and introduces a control system phase lag.